Infections with Salmonella enterica serovar Typhi (S. Typhi), S. Paratyphi A and S. Paratyphi B are responsible for the vast majority of enteric fevers globally and are a major public health concern. Additionally, Salmonella spp are category B pathogens that have the potential to be used as bio-terror weapons. The overall goal of this application is to advance the development of cross-protective vaccines against enteric fevers by identifying humoral and cell-mediated immune responses (CMI) which might play significant roles in protection. A secondary overall aim is to provide data to validate the S. Typhi /S. Paratyphi A bivalent vaccine approach to vaccination against enteric fevers. The proposed studies will take advantage of peripheral blood mononuclear cells and sera specimens obtained from subjects orally immunized with Ty21a and CVD 909 typhoid vaccines or with the attenuated CVD 1902 S. Paratyphi A candidate vaccine, from subjects challenged with wild-type (wt)-S. Typhi and wt-S. Paratyphi A and from typhoid and paratyphoid A patients in endemic areas. We will utilize the power of cutting edge immunologic platforms such as mass and conventional multichromatic flow cytometry analysis and sorting to characterize S. Typhi-, S. Paratyphi A-, and B-specific B([memory]) (BM), T [memory] , and T [regulatory] (T reg) responses, including their patterns of cytokine production and homing/chemokine receptor expression using a systems biology like approach. We will also perform immunoprofiling studies to identify genome-wide cross-reactive proteins that elicit serum antibody responses. Specifically, we propose to test the following hypotheses: (1) a defined set of CMI responses play a key role in cross-protection between S. Typhi and S. Paratyphi B infection in Ty21a-immunized subjects, (2) oral immunization of volunteers with an attenuated S. Paratyphi A vaccine strain (CVD 1902) or exposed to wt-S. Paratyphi A elicits a defined set of CMI responses against S. Paratyphi A antigens and infected cells that cross-react with S. Typhi and S. Paratyphi B antigens, and (3) oral immunization with the attenuated S. Paratyphi A CVD 1902 vaccine or exposure to wt-S. Paratyphi A (either in challenge studies or in endemic areas) elicits serum antibodies and BM, Beffector, and Breguiatory cells specific to S. Paratyphi A, as well as against Salmonella common antigens present in S. Typhi and S. Paratyphi B. These studies will contribute to the overall theme of this CETR application by providing novel and unique insights in humans that have the potential to advance the development of vaccine strategies to enteric fevers as well as other emerging enteric infections.